Immunization with vaccinia virus resulted in long-lasting protection against smallpox and was the successful approach used to eliminate natural smallpox infections worldwide. This accomplishment was achieved without a detailed understanding of human T cell responses to poxviruses. Due to the concern about the potential use of smallpox virus as a bioweapon, smallpox vaccination is currently being reintroduced. However, severe and life threatening complications from vaccination were associated with congenital or acquired T cell deficiencies, but not with congenital agammaglobulinemia. Considering the high incidence of side effects from current smallpox vaccine, the development of a safer, but equally effective vaccine is very important. Thus, it is important to have a detailed understanding of human T cell responses to poxviruses.
Vaccinia-specific CD4+ and CD8+ T cells have been detected in humans and the number of vaccinia virus-specific T cell responses to smallpox vaccine have been measured. Additionally, an intracellular cytokine staining assay was applied to quantitate and characterize vaccinia-specific T cells in mice. However, no T cell epitopes have been identified in humans or mice systems. One major obstacle is the size of the virus. Vaccinia is a large virus with an approximately 200-kbp DNA genome that has approximately 200 open reading frames.
In order to analyze T cell responses to licensed and experimental smallpox vaccines at the single cell level, it is essential to identify CD8+ T cell epitopes. In addition to emphasizing the importance of T cells in the immunity to smallpox, there is a critical need to develop new vaccines safe for use in T cell deficient populations. This information will be useful for the design and analyses of the immunogenicity of experimental vaccinia vaccines, and for basic studies of human T cell memory.